The present invention relates to an image forming apparatus such as a copying machine or a printer, and in particular, to an image forming apparatus capable of forming images on both sides of a sheet.
The image forming apparatus such as a copying machine or a printer is equipped with an image carrier, an image writing section, a developing unit, a sheet-feeding tray, a transfer section and a fixing section. Now, the constitution of the image forming apparatus will be explained as follows, referring to FIG. 18 which is a cross-sectional view showing the constitution of the image forming apparatus.
Automatic conveyance device 10 is a device to conduct conveyance for reading a document. A plurality of documents d each being under the condition that the front surface of the first page of the document faces upward are loaded on document loading section 11 which is for loading documents. Document d is fed out through roller 12a and roller 12b, and is conveyed to image reading section 20 through roller 13. Then, the document d whose images have been read in the image reading section 20 is reversed by reversing roller 14, to be ejected on sheet ejection tray 16 with its front surface facing downward.
The image reading section 20 scans the document optically to generate image data. An image surface of the document d is illuminated by light source 23, and its reflected light forms an image on a light-receiving surface of CCD 28 representing a photoelectric conversion means, through mirror 24, mirror 25, mirror 26 and combined optical system 27. Incidentally, when reading the document d by placing it on platen glass 21 so that the surface of the document d to be read may face downward, the optical system is moved along the platen glass 21 for the reading operation. Further, when reading the document d, while conveying it, the reading operation is conducted under the condition the light source 23 and the mirror 24 are fixed on second platen glass 22. Image data of the document d that has been read are sent to an image processing section (not shown) from CCD 28. In the mean time, when the document d is conveyed for its both sides by automatic conveyance device 10, the document d is reversed and conveyed to roller 13 again through reversing roller 14 after the front surface of the document d has been read, whereby, the rear surface of the document d is read by image reading section 20, and image data obtained through the reading are sent to an image processing section from CCD 28.
Transfer sheets P are loaded on sheet-feeding tray 30. Incidentally, though a single step of sheet-feeding tray 30 is provided in the structure in FIG. 18, it is normal that a plurality of sheet-feeding trays are provided so that transfer sheets having different sizes may be loaded.
Sheet supply section 40 supplies transfer sheets P to image forming section 60 from sheet-feeding tray 30. Transfer sheet P is fed out of the sheet-feeding tray 30 by conveyance roller 41, and is caused to hit a nip portion of a registration roller 43 through loop rollers 42 to be stopped temporarily, thereby, a skew of the transfer sheet P is corrected. Then, the transfer sheet P is conveyed to photoreceptor drum 61 of the image forming section 60 at prescribed timing. Further, the transfer sheet P is fed out of manual feed tray 31 by conveyance roller 41, and is conveyed to photoreceptor drum 61 of the image forming section 60 through the same process flow as in the foregoing.
Image writing section 50 is composed of a polygon mirror (not shown) that deflects a laser beam emitted from laser element 51 based on inputted image data. The deflected laser beam is caused by the polygon mirror to scan and is projected on photoreceptor drum 61 through a mirror. Owing to this, an electrostatic latent image is formed on the photoreceptor drum 61.
The image forming section 60 records the electrostatic latent image formed on the photoreceptor drum 61 on transfer sheet P through an electrophotographic system. First, when a laser beam emitted from laser diode 51 of the image writing section 50 is irradiated on the photoreceptor drum 61 charged evenly by charging section 67, an electrostatic latent image is formed. Then the electrostatic latent image formed on the photoreceptor drum 61 is developed by developing unit 62 to form a toner image on the photoreceptor drum 61. This toner image is transferred onto transfer sheet P by transfer section 63 that is provided below the photoreceptor drum 61. Then, transfer sheet P that is in contact with the photoreceptor drum 61 is separated by separating section 64. The transfer sheet P separated from the photoreceptor drum 61 is conveyed to fixing section 70 by conveyance mechanism 65.
The fixing section 70 fixes a toner image transferred onto transfer sheet P through heat and pressure. The toner image transferred onto transfer sheet P is fixed by heat and pressure exerted from fixing roller 71.
Sheet ejection section 80 ejects transfer sheet P on which the image has been fixed. Transfer sheet P on which the image has been fixed is ejected to sheet ejection tray 82 by sheet ejection roller 81. When forming images on both sides, transfer sheet P is conveyed downward by guide 83, after the image formed on the front surface is fixed, and the transfer sheet P is sent to reversing path 84. The transfer sheet P having entered the reversing path 84 is conveyed to reversing conveyance path 86 by reversing conveyance roller 85. The transfer sheet P having entered the reversing conveyance path 86 is conveyed again to image forming section 60 through sheet supply section 40.
Transfer sheet P is caused to hit a nip portion of the registration roller 43 through loop rollers 42 to be stopped temporarily, thereby, a skew of the transfer sheet P is corrected. Then, the transfer sheet P is conveyed to photoreceptor drum 61 of the image forming section 60 at prescribed timing.
On the image forming section 60, residual toner sticking to the image forming section 60 is removed by cleaning section 66, to be ready for the succeeding image forming. Under this condition, the transfer sheet P is conveyed to image forming section 60, and an image is formed on the other surface (rear surface). Then, the transfer sheet P separated from the photoreceptor drum 61 in the separation section 64 is sent again to fixing section 70 through conveyance mechanism 65 to be fixed. In this way, transfer sheet P on which image fixing on each of the front surface and the rear surface has been terminated is ejected to sheet ejection tray 82 by sheet ejection roller 81.
As stated above, the skew of a transfer sheet for the conveyance direction has been corrected by the registration roller 43 before image forming. With respect to the correction of the skew, there has been proposed a method wherein a pattern for measurement is written on the transfer sheet, then, the position of the pattern is detected to detect the positional shifting and the skew of the transfer sheet, and a position of an image to be formed on the reverse side is determined based on the results of the detection (for example, Patent Document 1).
As another method, there has been proposed a method wherein a mark is written on the surface of a transfer sheet, and when forming an image on the reverse side, a position of the mark is detected, then, a position of an image to be formed on the reverse side of the transfer sheet is determined based on the position of the mark detected and the position of the mark at the point of time when the mark was written, and further, the magnification of the image to be formed on the reverse side is changed (for example, Patent Document 2).
Further, as another method, there has been proposed a method wherein changes in outer dimensions of the transfer sheet generated when the image formed on the surface is fixed are obtained and thereby, the magnification of an image to be formed on the reverse side is changed (for example, Patent Document 3).
(Patent Document 1) JP-A No. 10-319674 (Hereinafter, JP-A refers to Japanese Patent Publication Open to Public Inspection)
(Patent Document 2) JP-A No. 2003-156974
(Patent Document 3) JP-A No. 2004-271926
However, actual transfer sheet P has no orthogonality, and corner angles fluctuate, depending on how sheets are cut. As shown in FIG. 19 (a), for example, an angle of a certain corner of transfer sheet P is different, and it is 89° for a certain corner and is 91° for another corner. If an angle of transfer sheet P fluctuates as stated above, there is sometimes an occasion where an image formed on the surface and an image formed on the reverse side do not agree in terms of position each other, even when a skew of transfer sheet P is corrected by the registration roller 43.
When forming an image on the front surface of transfer sheet P, for example, a side in the conveyance direction (a leading edge) of transfer sheet P is caused to hit a nip portion of the registration roller 43 so that a skew of the transfer sheet P for the conveyance direction is corrected, and after that, an image is formed on the surface of image forming section 30. After the image is formed on the surface, when an image is formed on a reverse side of the transfer sheet P, the transfer sheet P is reversed by reversing path 84, reversing and conveying roller 85 and reversing and conveying path 86, and is sent again to image forming section 6. Since the transfer sheet P is reversed as stated above, a side of transfer sheet P opposite to the leading edge (a trailing edge) is caused to hit a nip portion of the registration roller 43 as shown in FIG. 19 (c), and a skew of the transfer sheet P for the conveyance direction is corrected. After that, an image is formed on the reverse side by the image forming section 60.
However, since a skew angle of the leading edge is different from that of a side opposite to the leading edge (a trailing edge) in transfer sheet P as shown in FIG. 19 (a), an image formed on the surface is deviated from an image formed on the reverse side by the difference equivalent to the difference of the angle as shown in FIG. 19 (c), and it has been difficult to align an image on the surface with an image on the reverse side highly accurately. In other words, since transfer sheet P has no orthogonality with its each side skewing, even when different sides are caused to hit the registration roller 43 for correcting skews, the skew of transfer sheet P for conveyance direction before transferring images onto the surface is different from that before transferring images onto the reverse side. As a result, a position of the image on the surface is shifted from that of the image on the reverse side, resulting in difficulties of aligning images highly accurately.